The Matchbox Twenty frontman and his wife are are opening up about the debilitating illness that took over their lives 14 years ago.

Rocking out, side of stage, while her husband, Matchbox Twenty frontman Rob Thomas, performed in Chula Vista, California, Marisol Thomas may have appeared a gorgeous picture of health and happiness as she sang along to the band’s hit, “Disease.” But behind the smiles, the scene was a rare moment of reprieve from a debilitating 14-year fight against a disease which Rob says has transformed his perspective on life, robbed any thoughts of having more children and “inhabited” parts of his wife.

“It’s crazy how normal something abnormal can become,” Rob tells ET, during a rare interview with Marisol to promote Global Lyme Alliance’s 3rd Annual New York Gala -- where he will perform and Marisol is an honoree for her advocacy work for Lyme and other tick-borne diseases. “It’s like if one day you woke up and there was an alien living in your house and the first couple of weeks you’d be f**king freaked out, but after a while you’d name it and it would just be there. This sickness has been like this weird alien that’s moved into our life and inhabited my wife. Sometimes she’s just not herself. Watching the person you’re closest to become someone other than herself is so [hard].”

The couple has grappled with Marisol’s health battles for more than a decade, during which she visited countless specialists, desperate for accurate diagnosis and treatment. Showing symptoms of autoimmune disease, she was told she had Multiple Sclerosis one minute, Lupus the next and at one point, while lying in a hospital for days, she believed she had pancreatic cancer. Rob frustratingly recalls one specialist exploring the possibility of Lyme (a potentially fatal illness spread by ticks) years ago, but the common diagnostic test is inaccurate more than half the time.

However, after undergoing surgery to remove a lesion from her brain in 2015, doctors suspected and tested for the illness, eventually diagnosing eight tick-borne diseases -- including late stage Neurological Lyme Borreliosis, Babesiosis and Bartonella -- all of which induce different symptoms and require various treatments and doctors. The infections are also believed to have triggered Hashimoto’s disease and Lupus-like symptoms, as well as Atypical Trigeminal Neuralgia, nicknamed the “suicide disease,” because it attacks facial nerves and “makes you wish for death.”

“After brain surgery, everyone went, ‘You’re on the road to recovery -- you’re doing great,’” reflects Marisol, 41. “But that was actually the easiest part. The real battle began when we finally figured out what was going on with me.”

While Lyme can be cured with antibiotics if caught early, Marisol’s late-stage diagnosis and presence of multiple infections means it’s harder to manage. Her treatment is constantly tweaked according to which infection is flaring up most, and involves a mixture of pharmaceutical meds and holistic therapies, some of which are not easily obtainable due to being commercially unviable.

Once a driven “A-type,” who thrived in co-managing Rob’s career and passionately running their Sidewalk Angels Foundation, which assists animal rescue groups, she says Lyme has “stolen” her life and frequently traps her on the tour bus with symptoms including visual disturbance, numbness, seizures and constant pain. Being correctly diagnosed means she has better treatment, fewer seizures and more hours of feeling “semi-functional,” but the former model says there’s still a “long road ahead.”

“It’s like you’re standing still unable to participate as everything keeps going on without you, then you wake up the next day and the same thing happens,” she explains. “I don’t consider what I’m doing living. I’m existing … and fighting to hopefully one day live again. That’s a horrible place to be in and I have support to pursue it, but I know there are people who don’t. You become so sick of being sick and want nothing more than a moment away from it, so if I have a [good] moment I run with it. My mom and I call them happy moments and I do more in those hours than I ever would’ve before!”

Currently traveling with Rob on Matchbox Twenty’s “A Brief History of Everything” tour, such escapism has included visiting the California Mid-State Fair and hanging with actor pal Wilmer Valderrama.

Although the couple relish those precious periods of respite from Marisol’s physical symptoms, the mental toll of the illness is an ongoing battle, largely thanks to the “stigma” attached to Lyme, an illness which has increased 25-fold since national surveillance began in 1982, with more than 329,000 new cases each year in the U.S. alone. Marisol notes how Lyme sufferers often get accused of “exaggerating, faking or needing psychiatric help,” which has triggered some to consider or commit suicide.

Despite harsh comments and being accused of sharing her story for attention, she continues to advocate for tick-borne diseases and will be honored alongside menswear designer Joseph Abboud at Global Lyme Alliance’s Oct. 11 gala at Cipriani, 42nd Street, where fellow Lyme sufferers Yolanda Hadid and Aly Hilfiger are Honorary Event co-chairs. Rob, who performed at last year’s gala, will once again hit the stage alongside Chris Daughtry.

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Photo: Global Lyme Alliance / Getty Images

For Rob, watching his wife of almost 18 years somewhat vanish into a daily health battle has been a harrowing journey, which inspired powerful solo hits like 2009’s “Your Diamonds” and the poignant 2016 ballad “Pieces.”

The ongoing struggles have come with significant personal costs, like sacrificing family hopes, social lives and holidays.

Once contemplating having children together, Rob -- who has a 19-year-old son, Maison, from a previous relationship -- admits all thoughts of having more kids were long ago swallowed by illness. “For so long now, we’ve told ourselves that we never really wanted kids together that we don’t know if that’s even true,” he admits. “We’ve known for the last decade that because of Mari’s health, that was something we couldn’t do and you just resign yourself to the fact. If we can have Mari back healthy and she can have a normal life, then we don’t need a family to complete us because we have so much time to make up for.”

“The real personal cost has been [little things] like one of our dearest friends visiting us on the road last night and she couldn’t get off the bus to see him,” he continues. “She loves hanging with everyone, but she’s always one door away unable to be part of it. Or there’s holidays she has to let go by. I’ve never seen anybody love Christmas like this girl loves Christmas, so to watch it pass [is tough]. These little things make a year suddenly go by without us realizing.”

While Rob, 45, remains deeply dedicated to Marisol -- halting his 2015 solo tour after learning she needed surgery -- he admits the helplessness and guilt that comes with being her spouse can be arduous. Some relationships don’t survive the perils of the disease, with Hadid stating in her new book (Believe Me: My Battle With the Invisible Disability of Lyme Disease) that her sickness contributed to the collapse of her seven-year marriage to David Foster.

Rob often feels “survivor’s guilt and remorse,” for being healthy. “Often their day-to-day life makes you feel selfish just by doing normal things,” he says. “l’ll say, ‘Man, I had the worst show,’ then realize who I’m talking to and go, ‘I’m sorry, that’s the stupidest thing to say!’ You get perspective. I don’t think I have anything to complain about because I’ve seen how bad it can get for someone. The great thing about going to the Global Lyme Alliance benefit last year was seeing we’re not alone. People talk about their health struggles and what the family goes through and it’s normal to hear someone say, ‘I can’t tell you how much I thank my family for not just killing me!’ -- and think, ‘I totally get that.’”

“For public record, I’m not going to kill my wife!” he adds. “But they spend so much time trying to get better that, as a caretaker, you pick up the slack on everything else in their life. I get tired and frustrated, then I see a glimpse of Mari and we talk, recognize it, then keep going.”

Rob adds that having Marisol on tour has been a blessing because he’s always nearby, while still able to fulfil his duties with Matchbox Twenty.

The GRAMMY-nominated group, whose debut album Yourself or Someone Like You turns 21 next month, have loved being reunited following guitarist Kyle Cook’s brief fallout with Rob and departure from the band. Rob says the group is now stronger than ever.

“I don’t know that we’ve ever gotten along as well as we are now,” he shares. “We hashed everything out and have taken it to another level, where we enjoy each other’s company on and off stage. We’re constantly in communication, and even today, on a day off, we’re writing stupid, little things to each other. I have a solo record I’ve almost finished writing and will put out next year, but because of this tour, we know after that, we’re going to record new music and want to tour again.”

And, when that tour rolls around, Marisol naturally hopes to be side of stage for as many shows as she can. “On my good days, I make sure I’m there -- it’s my favorite thing!” she says. “We always kid how I’m a road chick from way back … music and touring has been a huge part of my life, so having moments where I can stand side of stage and see Rob do his thing and be better than ever is amazing. Those are my happy moments.”